However, the symptoms are extremely similar between historical records and current tests. This suggests that SCP-XXXX has been alive for at least 500 years. Yet, SCP-XXXX has shown no signs of aging.

Awkward phrasing. Why wouldn't we just say it's a coincidence if there are no records of the entity anywhere?

(A note for critics: I will be adding more addendums later.)

I don't see the need for any more addenda.

Overall, it really doesn't give a real reason why they believe it's connected to the dancing plague. I don't see how or why it was recovered in a gymnasium, and I just don't get any appeal to this concept. Personally, I think it could work if more of a story was added to it.

Hey, thanks for the feedback, I really needed it because I'm sort of new to being an SCP author.
Anyway, what I was trying to do with people being attracted to the SCP was that it would eventually become an unstoppable amalgamation of floating/dancing people until everyone was dancing. The small amount of time that it takes for the SCP to take control is why staff members need to be rotated… Maybe I should up the time for that effect? Or get rid of it entirely?
I'll expand on and change the SCP's connection to the dancing plague so that it's more solid, I really want that to happen.
Also, sorry for all of the awkward framing, like I said I'm a bit new to this, so my wordage might not be the most proficient that it could be.